Crocodiles and their ancient relatives may have survived the asteroid impact that wiped out most dinosaurs 66 million years ago because they were already adapted to warmer climates, according to a new study. By analyzing more than 7,000 fossils and records of over 400 living and extinct species, researchers found that crocodilian lineages generally tracked suitable temperatures throughout their 250 million-year history. After the impact, tropical and subtropical regions may have remained habitable refuges for surviving populations.
Why crocodiles survived the dinosaur extinction
Scientists led by researchers at UCL (University College London) combined fossil evidence with computer models reconstructing Earth’s climate and geography over the past 245 million years. The results suggest that crocodiles and their relatives typically followed temperate and warm environments rather than adapting to dramatically different temperatures.
As the planet warmed, crocodilian lineages expanded into new regions, at times reaching as far south as Antarctica. When temperatures fell, their ranges contracted toward the tropics.
The researchers used climate simulations to investigate the decades and centuries after the Chicxulub asteroid impact 66 million years ago. The impact caused major environmental disruption and the extinction of most dinosaurs. The simulations included possible effects such as sunlight being blocked by dust and ash, along with increases in atmospheric carbon dioxide associated with volcanic eruptions.
When sunlight was reduced by 5% or 10%, warm regions near the equator remained suitable for crocodilian relatives. These areas may have provided refuges where surviving populations could persist and eventually recover. By contrast, a more severe scenario involving a 15% reduction in sunlight made those regions unsuitable. The researchers noted that this extreme level of sunlight decline is now considered unlikely.
Other simulated changes, including increased atmospheric carbon dioxide, did not significantly alter the availability of suitable crocodilian habitat.
Previous modeling by members of the same research team found that habitat suitable for non-avian dinosaurs shrank much more severely after the impact.
Lead author Dr Alfio Alessandro Chiarenza of UCL Earth Sciences said: “The asteroid did not make the entire planet equally hostile to any group.”
“Crocodilians have long favored warmer environments and may have placed their survivors in areas where suitable climate is best, such as near the equator. These refugia may have protected populations that then recovered and spread outward.”
The findings offer a possible explanation for how a group of animals that emerged around the same time as dinosaurs survived two of Earth’s five major mass extinctions.
Ancient crocodiles were more diverse
Modern crocodiles and their relatives share many recognizable features, but their evolutionary history includes a much wider range of body shapes, sizes and lifestyles.
The broader crocodilian lineage once included heavily armored herbivores, entirely terrestrial predators and giant animals weighing more than a ton. One notable example was Sarcosuchus, a huge predator often known as a “super crocodile” that could reach up to 12 meters in length.
Despite this diversity, the researchers found that the relationship between crocodilian lineages and climate remained surprisingly consistent.
Dr Chiarenza said: “Although crocodiles are often referred to as living fossils, their ancestors and relatives have undergone significant changes throughout their history, ranging from giant crocodiles the size of a bus to those that can swallow people.”
“Our study shows that despite this diversity, one thing that was surprisingly consistent was the relationship with climate.”
What fossils reveal about 245 million years of climate history
To investigate that relationship, the scientists reconstructed the temperature and precipitation preferences of 400 living and extinct crocodiles and their ancient relatives. They combined fossil evidence, evolutionary family trees and detailed maps showing where modern crocodilian species are found.
The team then compared those preferences with reconstructions of ancient geography and climate to determine where suitable habitat existed at different points in Earth’s history.
A clear pattern emerged: crocodilian relatives generally followed warmer environments as the planet’s climate changed. Their ranges expanded during warmer periods and contracted when temperatures cooled.
About 50 million years ago, during the particularly warm early Eocene, global temperatures were approximately 10°C higher than today, and the polar regions had no permanent ice. Crocodilian relatives lived at high latitudes in North America, Europe and even Antarctica.
Over the following several million years, global cooling gradually reduced the area where they could thrive, and their geographic range contracted toward warmer tropical regions.
Two important exceptions
Although the preference for warmer environments was consistent across most of the evolutionary record, the researchers identified two significant exceptions.
During the Cretaceous Maximum, about 94 million years ago, global temperatures rose to approximately 14°C above modern levels. One lineage in the crocodilian family tree evolved the ability to tolerate hotter conditions than other lineages could withstand.
A second adaptation occurred in Australia about 34 million years ago. As prolonged cooling reduced precipitation and dried the continent, extinct crocodilian groups adapted to increasingly arid, desert-like environments.
These examples show that some crocodilian relatives could evolve to cope with different climates, even though most crocodilians continued to track the warmer conditions they preferred.
A survival strategy that may now be a weakness
The researchers stressed that favorable temperatures were probably only one reason crocodiles survived the mass extinction. Other traits may also have helped, including relatively small body sizes, the ability to live partially underwater, a flexible diet and moderate energy requirements.
As ectothermic, or “cold-blooded,” animals, crocodiles require less food to sustain themselves than animals that generate their own body heat.
The findings also raise questions about how crocodiles will respond to modern climate change. For millions of years, moving to areas with suitable temperatures appears to have been one of their main responses to a changing planet. Today, however, cities, farmland and roads increasingly restrict the ability of wild animals to move between habitats.
Professor Philip Mannion of UCL Geosciences said: “These findings have implications for the species today. If crocodiles have coped with climate change in the past primarily by moving to suitable habitats, then as the world warms, species now need room to move, but this is becoming more difficult as suitable areas for roads, farms and cities disappear.”
The research was funded by the Royal Society, the Leverhulme Trust and the National Environmental Research Council (NERC).
Source: www.sciencedaily.com